With the increasing demand for processing heavier crude and higher conversion of crude, processes for treating heavy and refractory feedstock play a more important role in refineries. Owing to its relatively simple technique and lower investment, delayed coker is becoming more and important for processing heavy oil and residual oil. Full-range distillates including naphtha as dominate liquid product is obtained from the delayed coker. Since coker naphtha, also called coker gasoline is unsuitable feedstock for downstream process due to its high content of unsaturated hydrocarbons and impurities such as sulfur, nitrogen and etc as well as poor stability, it cannot be directly used as feedstock for consequent procedures. It must be hydrofined to improve its stability and to remove the impurities such that it can be widely used, for example, as a feedstock for ethylene production, a feedstock for synthetic ammonia and a feedstock for reforming, and as chemical light oils and automotive fuels.
Fluid Catalytic cracking (FCC) is also an important means for deep processing heavy oils and residual oils. Different from delayed coking, FCC feedstock, generally hydroprotreated, has a better crackability. Similar to delayed coking, FCC products such as naphtha and LCO have a high level of unsaturated hydrocarbons and a certain amount of impurities such as sulfur, nitrogen, etc.
Some naphtha fraction from other industrial processes such as pyrolysis also has the similar properties as above.
The above-mentioned naphtha fraction with poor quality from coker, FCC and pyrolysis processes is stated as inferior naphtha (gasoline) fraction in the present patent.
Industrial practice shows that one of the major problems to affect operation, the hydrotreater for inferior naphtha has to be shutdown due to the short-term increase of pressure drop across the layers of the hydrogenation catalyst bed, which is mainly caused by the polymerization of the dienes in the feedstock. Under higher temperature, olefins, dienes and the like in the feedstock are prone to form macromolecular organic compounds via Diels-Alder recycle reaction or polymerize reaction, and even further condensed to coke. These coking reactions mainly occurred at such parts as high-temperature heat exchanger, heating furnace and the top of the reactor. Frequent shutdown caused by coking severely disturbs the normal unit operation.
In the prior art for hydrotreating inferior naphtha fraction, although outlet temperature materials from heat exchanger and heating furnace is not high, the higher wall temperature leads heat exchanger and heating furnace to severe coking. The coke in the heat exchanger and the heating furnace may sometimes enter the reactor with the feedstock and deposit on the top of reactor catalyst bed, which further accelerates the blocking of the catalyst bed.
U.S. Pat. No. 4,113,603 discloses a two-stage hydrofining process for treating dienes and sulfides in pyrolysis gasoline, wherein a Ni—W catalyst is used in the first stage to remove thiol, and a noble metal palladium catalyst is used in the second stage to remove dienes. This process is complex. Since the noble catalyst is intolerant to sulfur and the reaction temperature is low, such process is not suitable for hydrogenating coke naphtha.
CN200710012091.0 discloses a method of improving the operation period of the device for hydrotreating inferior naphtha, wherein an additional reactor is set before the heating furnace so that the inferior naphtha is first subjected to a reaction at a low temperature for selectively hydrogenating dienes in the additional reactor, and then passed into a main reactor to undergo hydrogenation to remove the impurities such as sulfur and nitrogen as well as olefin saturation reaction. During this method, the feedstock of the first reactor needs to be warmed to the desired temperature in a heat exchanger. Although the required inlet temperature of the first reactor is low, the tube wall of the heat exchanger has a very high temperature (i.e. the temperature of the outlet materials of the second reactor, normally above 300). Accordingly, the heat exchanger is still subject to a coking problem.